I've already discussed the benefits of a weak notrump, so I'll just recap with a quick summary here:
- Lets you open far more frequently than a strong notrump
- Conveys a lot of information to partner
- Hides information from opponents
- Preempts more than a 1m opening
- Offers a well-defined and thorough response framework
- No worrying about opening 1M or 1NT with a 5-card major and 15–17 HCP
Because Responder needs more strength to respond, and since opening 1♦ guarantees finding a 4-4 major fit (if no interference), Eastern Mysticism prefers opening 1♦ with both majors even if balanced.
Standard bidders considering a weak notrump may worry about getting doubled and going for a number. In our experience, that happens about as often as it does to strong notrumpers—almost never. I'd say that's for two reasons primarily:
- opponents rarely want to try and set a 1-level contract, and
- we have a good escape sequence (DONT) if we are doubled.
In addition, Eastern Mysticism avoids this potential pitfall by using a variable notrump. In first or second seat, there's no reason to suspect an adverse lie of opponents' strength. Opening weak (11–13 HCP) is fine. But opening in third or fourth seat makes it much more likely that opponents' values are sitting over the opener. In these seats, Eastern Mysticism requires 13–15 HCP to open 1NT.
Naturally, we play major-suit transfers. We also play an Extended Garbage Stayman that gives Responder as many options as possible to find a 4-4 or even 5-3 major fit. (When Responder has a 5-card major, that is; Opener will never have a 5-card major. Which means we don't have to bother with Puppet Stayman.)
All these conventions cover nearly every imaginable shape and strength for Responder, so we don't need 3-level responses to show two-suiters (the most common use). With nothing better to do, we use the 3-level for Keycard Blackwood… I think it's come up once or twice in the past decade.